The retinoblastoma susceptibility gene, RB, is a prototypic tumor suppressor gene. It has been shown to be involved fundamentally in the genesis of the intraocular childhood tumor retinoblastoma, as well as in a number of other human tumors. For the past eight years, we have isolated the retinoblastoma gene - the first human tumor suppressor gene, then shown that Rb gene replacement in many cultured tumor cells containing mutated RB alleles suppresses the cells' neoplastic properties. These results provided an initial basis for considering treatment of clinical cancer by replacing the wild-type Rb gene. To extend this cancer suppression concept to intact tumors in vivo, we have established a line of mice harboring RB mutations in the germline. Mice heterozygous for the mutant gene predictably develop specific cancers of the pituitary gland intermediate lobe with nearly 100% penetrance which mimics the natural human tumor formation. All these work set up a stage for us to finally test whether the tumor suppressor genes can be of use in clinical application. The experiment described in this proposal is an essential step before clinical applications can be sought. First, viral or nonviral vectors for efficient Rb gene delivery will be developed. Second, cell lines derived from mouse pituitary tumors will be established. Then the Rb gene delivery vectors will be tested, ex vivo and in vivo, using in situ labeling to demonstrate RB expression in targeted tumors, and magnetic resonance imaging and tumor markers to determine the effects of therapy on tumor growth. Third, the molecular events of tumor progression will be examined to define precisely the best therapeutic window for intervention. The concept of cancer suppression by the RB gene has not yet been taken to its ultimate and practical conclusion, and only by in vivo studies can it be taken there. The results from the proposed studies will provide a groundwork for treating human cancer not only with the retinoblastoma gene but also with a large number of newly discovered tumor suppressor genes.